Rigid or semi-rigid elongate members, such as spinal rods, may be mounted to the spinal column in order to stabilize or immobilize vertebrae of the spinal column for a variety of purposes. For instance, spinal rods are often secured to adjacent vertebral bodies via anchor members in order to promote fusion of the two vertebrae as a treatment for degenerative disc disease, spondylolisthesis, spinal stenosis, fractures of the vertebrae, and other conditions. Limiting or preventing motion of the vertebrae promotes the healing process. By removal of the disc positioned between the vertebrae and limiting motion between the vertebrae, the adjacent boney surfaces are allowed to grow into one another and fuse together. Fusion devices may also be placed between the two immobilized vertebrae in order to facilitate the process of fusion.
When stabilizing portions of the spinal column, and in particular the cervical region of the spine, it is sometimes necessary to immobilize the skull in addition to vertebrae. The same elongate rigid structures used to link and stabilize the vertebrae may therefore be secured to the skull in order to keep the skull in an appropriate spatial relationship with respect to the spinal column. However, since the anatomy and thickness of the skull and its surrounding tissues are very different than those of the vertebrae and their surrounding tissues, the elongate rigid structures must be anchored to the skull in a different manner than that used for the vertebrae.
For instance, in many spinal stabilization procedures elongate rods made of titanium or other materials are placed adjacent to the posterior side of the spine and anchored in place using screws connected to some type of coupling assembly. Examples of coupling assemblies for posterior fixation systems are disclosed in U.S. Pat. No. 7,141,051; U.S. Published Application No. 2008/0045955; and U.S. Published Application No. 2007/0225711. The screws used to anchor these devices and other coupling assemblies are often relatively long, and are mounted to the pedicle area of the vertebrae with the shanks of the screws penetrating deep into the vertebral body. The yoke portion or receiving member of the coupling assembly that is coupled to the screw and receives the spinal rod is nested between outwardly-extending boney processes so that the height of the yoke is not noticeable.
When spinal rods are mounted to the skull, however, it is undesirable to use long screws and large coupling assemblies. Many coupling assemblies also would prove unduly cumbersome if mounted directly to the skull, and may even protrude significantly from the back of the head. In addition, the occipital region, which juts out at the base of the skull, typically is the site at which to mount an internal fixation system, requiring that spinal rods connected thereto be severely bent in order to be positioned along the occipital region and be connected to the occipital region in a manner similar to the connection to the vertebrae.
Previous systems for coupling spinal rods and other elongate stabilization devices to the skull vary. However, most systems utilize a plate mounted to the occipital region of the skull that attaches to a rod, cable, wire, plate, or screw mounted to a region of the spine. In most spinal rod systems, two spinal rods are positioned generally parallel to the surface of the plate and then secured thereto by a bracket, yoke, or other receiving member, such as a U-shaped receiving member. The plates are mounted to the skull with several small screws disposed along the full length and width of the plate. Since the base of the skull angles inward toward the spine, the plates mounted to the skull are not parallel to the posterior surfaces of the vertebrae, and the spinal rods must be bent significantly away from the vertebrae in order to reach the occipital region in an orientation so that they may be mounted to the plate. For instance, the bending of spinal rods in order for them to properly be received relative to an occipital plate is shown in the devices of FIGS. 1, 2, and 18 of U.S. Published Application No. 2004/0153070. In that device, spinal rods mounted along the vertebrae must be manipulated in order to fit precisely into receiving mechanisms aligned along the sides of a plate designed to be fixed to the occipital region of the skull. This bending of the rod can fatigue the rod material, and also makes it difficult to reposition the elements of the stabilization system.
Even attempts to provide occipital plate devices with adjustability in order to accommodate spinal rods of various orientations still generally require significant manipulation and bending of spinal rods before they can be secured to the plate structure. For instance, U.S. Pat. No. 6,902,565 discloses a plate designed to be mounted to the occipital region of the skull by a plurality of short expansion head screws. The plate receives a pair of rods that may be further mounted to one or more vertebrae. In many cases these rods are pre-bent so that the majority of the rods may be positioned parallel to the spine, with the ends bent transversely in order to be secured to the plate by a clamp plate or bracket. Some embodiments include plates that are bent in order to receive the rods that are parallel to the spine.
U.S. Published Application No. 2008/0051783 discloses a plate device having a pair of u-shaped rod receiving members that protrude from lateral wings of the plate. The wings may be shifted laterally and medially, and the rod receiving members may rotate to adjust the direction in which a connecting member (such as a spinal rod) is received. The spinal rods must be positioned so that they are generally parallel to the plate surface in order to fit into the rod receiving members. Therefore, the ends of the rod must be bent away from the axis of the spine, which is not parallel to the plate surface, and into the u-shaped channels of the receiving members.
U.S. Pat. No. 6,524,315 discloses a plate secured to the bone by a plurality of screws. The plate is fitted with slotted bolts designed for receiving a rod or cable. The base of the slotted bolt is recessed in the plate at its base. A support platform may be fitted over the bolt to help hold the rod or cable. A nut fastens over the threaded end of the slotted bolt to trap the rod or cable within the bolt, securing it to the plate. The bolt may be rotated to adjust the direction of the rod or cable.
U.S. Published Application 2007/0233119 discloses a plate device with polyaxial connector head assemblies including a connector body that receives a spinal rod and a connector head pivotably connected to the connector body and configured to be secured to the plate so that the connector assemblies provide limited polyaxial movement of the spinal rods with respect to the plate. However, the coupling heads are relatively bulky and still hold the spinal rods relatively parallel to the plate surface.
Even more adjustable occipital plates have various shortcomings. In U.S. Published Application 2007/0118121, a fixation plate includes a laterally extending arm coupled to a pair of spinal rods by sliding links that are able to slide along and pivot about the arms. However, ends of the spinal rods are held relatively close to the fixation plate, and positioning of the sliding links is limited to sliding and pivoting along the fixed laterally extending arms. Further, the sliding links are locked in place by clamping together top and bottom portions thereof with a set screw positioned at a distance from the laterally extending arm, which compresses a rounded portion of the link about the arm in order to inhibit sliding. However, the locking force between the sliding links and the laterally extending arms may not be able to prevent pivoting or sliding of the spinal rods relative to the fixation plate when sufficient force is applied.
U.S. Pat. No. 7,901,433 also discloses an adjustable occipital plate system that permits spinal rods to be positioned at various angles with respect to the plane of the occipital plate. The horseshoe-shaped plate has a lateral arm extending from each side, with a variable connector securing each spinal rod to a lateral arm of the plate system. A gap separates the two lateral arms, and may allow some twisting of the plate arms and spinal rods. In addition, the horseshoe-shape of the plate may not allow all of the bone screws to be driven into the thickest and hardest bone which is typically in the central area of the occipital region of the skull. The horseshoe-plate of the '433 patent also will have to be bent because of its substantially flat configuration and the non-flat or curved configuration of the skull's occipital region. Bending of the relatively narrow, curved arms of the plate will undesirably further weaken the plate. Additionally, bending the plate and the rods involves trial-and-error and, as such, is typically a very time consuming process. This is particularly challenging with these types of occipital plate and spinal rod assembles where during surgery, the patient's occiput and cervical vertebrae are typically oriented one way relative to each other with the patient supported on an operating table, and need to be oriented another way relative to each other for final fixation of the assembly to the occiput and the cervical vertebrae. The connectors of the '433 patent hold the spinal rods below or even with the lateral arms, and thus relatively close to the spine. However, this can create interference with the cervical vertebrae immediately below the occiput (i.e., the C1 and C2 vertebrae), and potentially the brain stem where it may be exposed due to damage to surrounding bone.